Question

我通过创建3个表（classes, lectures, taking）来创建数据库，然后依次更改表以添加foreign key。

但是，我不断收到以下错误： ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint

我似乎未发现代码有任何问题，所以可能是什么问题？？（我使用的是mySQL Workbench。我尝试将复制和粘贴作为txt文件，因为我虽然可能是排序规则问题，但仍然会出现相同的问题）

代码如下：

CREATE TABLE classes (
    course_id VARCHAR(8),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    PRIMARY KEY (course_id, classes_id, semester, year)
);

CREATE TABLE taking (
    student_id VARCHAR(8),
    course_id VARCHAR(10),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    grade char(1),
    PRIMARY KEY (student_id, course_id, semester, year)
);

CREATE TABLE lectures (
    professor_id VARCHAR(8),
    course_id VARCHAR(10),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    PRIMARY KEY (professor_id, course_id, semester, year)
);

ALTER TABLE taking ADD CONSTRAINT consTAKE3 FOREIGN KEY(classes_id) REFERENCES classes(classes_id) ON DELETE CASCADE;
ALTER TABLE taking ADD CONSTRAINT consTAKE4 FOREIGN KEY(semester) REFERENCES classes(semester) ON DELETE CASCADE;
ALTER TABLE taking ADD CONSTRAINT consTAKE5 FOREIGN KEY(year) REFERENCES classes(year) ON DELETE CASCADE;

ALTER TABLE lectures ADD CONSTRAINT consLEC3 FOREIGN KEY(classes_id) REFERENCES classes(classes_id) ON DELETE CASCADE;
ALTER TABLE lectures ADD CONSTRAINT consLEC4 FOREIGN KEY(semester) REFERENCES classes(semester) ON DELETE CASCADE;
ALTER TABLE lectures ADD CONSTRAINT consLEC5 FOREIGN KEY(year) REFERENCES classes(year) ON DELETE CASCADE;

错误如下：

ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE3' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE4' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE5' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC3' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC4' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC5' in the referenced table 'classes'

现在，我正在尝试使用cmd从ysql –u root –p执行此操作。尽管如此，同样的问题仍在发生。

Answer 1

由于所有表中都有一个复合主键，因此所有引用也必须包括表中的所有复合键列才能正确映射。

除非每个表的复合主键列上都有唯一的约束/索引。

要在表中创建关系，请对dbo.taking进行尝试，对其他表也要采用类似的方法

ALTER TABLE dbo.taking 
  ADD CONSTRAINT FK_taking_classes
  FOREIGN KEY(course_id, classes_id, semester, year) REFERENCES classes(course_id, 
classes_id, semester, year)

Answer 2

两个表中course_id的字符长度不同，这导致未创建FK。请在与varchar(8)相同的taking表中将其设置为classes。肯定会解决问题。通过使用提供的查询，我已经创建了表，并且也有FK。

然后遵循上述查询：

由于所有表中都有复合主键，因此所有引用还必须在表格以正确映射。

除非您在复合主键上具有唯一的约束/索引每个表格上的列。

要在表中创建关系，请对dbo.taking进行尝试，请按照其他表也有类似的方法
  ALTER TABLE dbo.taking 
  ADD CONSTRAINT FK_taking_classes
  FOREIGN KEY(course_id, classes_id, semester, year) REFERENCES classes(course_id, 
  classes_id, semester, year)

错误1822（HY000）：无法添加外键约束。 SQL中约束问题的索引缺失

2 个答案: